After so many posts from the volcanic island of Oahu, you wouldn’t think there was much left, but I couldn’t overlook the most famous volcano of all, although technically Diamond Head (Fig. 1) is a tuff cone like Koko Crater. This brief post is going to examine the internal structure of one of its limbs, on the seaward side.
The lighter color of the rocks in Fig. 2, compared to what we saw at Koko Crater or elsewhere on Oahu, suggests that the underlying magma chamber was depleted of mafic minerals. Dark hues associated with basalt are caused by minerals like plagioclase feldspar, amphibole and pyroxene, and biotite mica. The lighter color of the road cut (fresh and unweathered) suggests that the magma contained felsic minerals like albite and orthoclase feldspar, quartz, and muscovite mica. I could be completely wrong about this but there is no doubt that the rocks in Fig. 2 are not dark gray or black…
My hypothesis is consistent with what is known about the crystallization sequence of minerals from a melt and the resulting viscosity of igneous rocks. Mafic minerals and the lava they form have low viscosity and flow readily, as we’ve all seen in videos of eruptions on the island of Hawaii. These magmas bubble, flow, shoot fire into the air, and release pressure easily. However, felsic minerals (especially quartz) are sticky and have high viscosity, which causes them to resist flow, contain gasses, and eventually explode spectacularly (e.g. Mt. St. Helens).
I think the Diamond Head vent (i.e. volcano) tapped a part of the magma chamber that had already lost most of its mafic minerals, but it wasn’t as explosive as Mount St. Helens.
Another surprising feature I saw along the seaward margin of the Diamond Head tuff cone was a set of vertical joints filled with reddish rock (Fig. 4).
It is important to remember that the entire island of Oahu was constructed by magma escaping through innumerable fissures like those seen in Fig. 4, at first creating thick lava sequences deep beneath the Pacific Ocean’s surface, then flowing through breaks in the jumbled mass of previous flows. By the time the pile of basalt reached the water’s surface to form Oahu, the magma chamber was running out of gas (so to speak), and the lava was thicker and more viscous.
Diamond Head and Koko Craters were the result of these last gasps.
This post concludes my visit to Oahu, an island that rose from the sea less than five million years ago, formed by a huge magma chamber that was created when the Pacific plate slid over an upper mantle hot spot so concentrated that it melted ocean crust an constructed the Hawaiian archipelago, more than 1500 miles long.
I encourage anyone reading this post to explore the amazing story of this new land as it was populated by plants and animals, culminating in the incredible story of how Polynesian culture reached this remote land…